Microemulsions temperature dependence

Muller, N. Errors in micellization enthalpies from temperature dependence of critical micelle concentrations. In Micellization, solubilization, and microemulsions. Vol. l,p. 229, Mittal, K. L. (ed.). New York - London Plenum 1977... 

The importance of a surfactant - rich phase, particularly a lamellar one, to detergency performance was noted for liquid soils such as C16 and mineral oil (3.6). Videomicroscopy experiments indicated that middle phase microemulsion formation for C12E04 and Cjg was enhanced at 30 °C, while at 18 °C, oil - in - water, and at 40 °C, water - in - oil microemulsions were found to form at the oil - bath interface (3.6). A strong temperature dependence of liquid soil removal by lamellar liquid crystals, attributed to viscosity effects, has been noted for surfactant - soil systems where a middle - phase microemulsion was not formed (10). 

Figure 7. The percolation behavior in AOT-water-decane microemulsion (17.5 21.3 61.2 vol%) is manifested by the temperature dependences of the static dielectric permittivity es (A left axis) and conductivity r (Q right axis). Toa is the temperature of the percolation onset Tp is the temperature of the percolation threshold. Insets are schematic presentations of the microemulsion structure far below percolation and at the percolation onset. (Reproduced with permission from Ref. 149. Copyright 1998, Elsevier Science B.V.)...

The dielectric relaxation properties in a sodium bis(2-ethylhexyl) sulfosuc-cinate (AOT)-water-decane microemulsion near the percolation temperature threshold have been investigated in a broad temperature region [47,143,147]. The dielectric measurements of ionic microemulsions were carried out using the TDS in a time window with a total time interval of 1 ps. It was found that the system exhibits a complex nonexponential relaxation behavior that is strongly temperature-dependent (Figure 8). 

Figure 8. Three-dimensional plots of the frequency and temperature dependence of the dielectric permittivity s (a) and dielectric losses e" (b) for AOT-water-decane microemulsion. (Reproduced with permission from Ref. 143. Copyright 1995, The American Physical Society.)...

The third relaxation process is located in the low-frequency region and the temperature interval 50°C to 100°C. The amplitude of this process essentially decreases when the frequency increases, and the maximum of the dielectric permittivity versus temperature has almost no temperature dependence (Fig 15). Finally, the low-frequency ac-conductivity ct demonstrates an S-shape dependency with increasing temperature (Fig. 16), which is typical of percolation [2,143,154]. Note in this regard that at the lowest-frequency limit of the covered frequency band the ac-conductivity can be associated with dc-conductivity cio usually measured at a fixed frequency by traditional conductometry. The dielectric relaxation process here is due to percolation of the apparent dipole moment excitation within the developed fractal structure of the connected pores [153,154,156]. This excitation is associated with the selfdiffusion of the charge carriers in the porous net. Note that as distinct from dynamic percolation in ionic microemulsions, the percolation in porous glasses appears via the transport of the excitation through the geometrical static fractal structure of the porous medium. 

The temperature dependence Of the microemulsion was described by reporting the behavior of both ande Ve (loss tangent) against decreasing temperature. The latter parameters were determined with an average uncertainty of 5 and 10, respectively. 

As a summary, the change of the structure due to a small increase of temperature is the formation of a water core resulting from a partial dehydration of the oxyethylene sites. Coorelative-ly the addition of water promotes the transition between lamellar aggregate and globular microemulsion, till the area per polar head reaches a maximum value which is temperature dependent. 

With nonionic surfactants, both types of microemulsions can be formed, depending on the conditions. With such systems, temperature is the most cracial factor as the solubility of surfactant in water or oil is temperature-dependent. Microemulsions prepared using nonionic surfactants will have a limited temperature range. 

Nonaqueous microemulsions with nonionic surfactants have been studied. The C12E4 surfactant was found to stabilize microemulsions of formamide and dodecane [138], The ternary phase diagrams were studied at different temperatures and the solubilization of hydrocarbon was shown to be very temperature dependent (Figure 6.7). It was also observed that the temperature intervals of the three-phase regions are dependent on the hydrocarbon used larger aliphatic hydrocarbons... 

In the previous section a quinary ionic microemulsion was timed through the phase inversion by adding a short-chain alcohol as a non-ionic co-surfactant to a single-tailed ionic surfactant. In the following the short-chain alcohol is replaced by an ordinary long-chain non-ionic surfactant. It was discussed above that the temperature dependence of the phase behaviour of ionic (see Section 1.2.4) and non-ionic microemulsions (see Section 1.2.1) is inverse. Thus, one can expect that at a certain ratio 8 of non-ionic and ionic surfactants the inverse temperature trends compensate so that a temperature-insensitive microemulsion forms. It goes without saying that this property is extremely relevant in technical applications, where often mixtures of non-ionic and ionic surfactants are used. 

Figure 1.14(a) shows the phase prism of the system water-oil-non-ionic surfactant (already shown in Fig. 1.3) together with the temperature dependence of the interfacial tensions (Fig. 1.14(b)). As discussed in Section 1.2.1, at low temperatures, non-ionic surfactants mainly dissolve in the aqueous phase and form an oil-in-water (o/w) microemulsion (a) that coexists with an oil-excess phase (b). Thus, for temperatures below the temperature T the interfacial tension microemulsion separates into two phases (a) and (c) at the temperature T) which, in turn, leads to the appearance of the three-phase body. Thus, three different interfacial tensions occur within the three-phase body, namely the interfacial tension between the water-rich and the surfactant-rich phase crac, between the oil-rich and the surfactant-rich phase oyc, and between the water-rich and the oil-rich phase uab. However, the latter can only be measured if most of the surfactant-rich middle phase (c) is removed, which then floats as a lens at the water/oil interface. Increasing the temperature one observes that the three-phase body vanishes at the temperature Tu, where a water-in-oil (w/o) microemulsion is formed by the combination of the two phases (c) and (b). Therefore, at temperatures above Tu the interfacial tension crab refers to the interface between a w/o-microemulsion and a water-rich excess phase. 

In order to use electron microscopy to visualise the microemulsion structure, the problem of the fixation of the liquid mixtures has to be solved. The method of choice is to solidify the microemulsion structure via cryofixation. However, given that the phase behaviour as well as the curvature of the amphiphilic film (see Fig. 1.18) and with it the microstructure of most micro emulsions show a strong temperature-dependence it has to be ensured that the cooling rate should be as high (>104 K/s) and the reorganisation kinetics of the microstructure as slow as possible. 

If large quantities are used for technical processes, e.g. for cleaning, the recovery and reuse of the microemulsion or at least of a considerable amount of the most expensive components is desired. Therefore, strategies are needed to separate contaminants from the organic microemulsion components. Separation is usually more complicated than from ordinary solvents and often requires several steps [39, 40]. In particular, the separation of waste materials from the surfactants is usually very difficult or often even impossible. The temperature-dependent phase behaviour of bicontinuous microemulsions, however, can sometimes be beneficially used for separation [41]. Easy separation, at least from the unpolar solvent, can be achieved from microemulsions with supercritical liquids [42]. 

Titania. Titania powders are used as pigments, catalytic supports, membranes, opacifiers, photocatalysts and fillers in industrial applications . Titania particles have been prepared by a number of methods, such as hydrolysis, sol-gel, microemulsion and hydrothermal synthesis. Titania exists naturally in two tetragonal forms, the metastable phase anatase, and the stable phase rutile. On heat treatment, anatase transforms into rutile. The phase transition temperature depends on the starting materials and the preparation procedure. 

The third effort in the early development in microemulsion science originated with Saito and Shinoda [18,19] in Japan. In their studies on the temperature-dependent behavior of water-hydrocarbon-polyethylene glycol alkyl (aryl) ether systems, a relationship was observed between the cloud point of the surfactant and the solubilization of the hydrocarbon. For aliphatic hydrocarbons it was found that the solubilization of hydrocarbon... 

Figure 9 Relative self-diffusion coefficients of water and oil as a function of the oil volume fraction

For W/O microemulsions, deviations from hard-sphere behavior are sometimes observed. The reason for this is that here there is a stronger tendency for attractive interactions and the formation of anisometric shapes. Here viscosity data can be used to determine the shape of the corresponding aggregates or to extract information regarding the interaction potential that exists between the droplets. From temperature-dependent experiments the binding enthalpy of the droplets can be determined. 

Yet another related approach is to use the strong temperature dependence of microemulsions based on nonionic surfactants. After the reaction is completed in a one-phase microemulsion, the temperature is raised (or lowered) so that a two-phase system forms, consisting of microemulsion in equilibrium with excess water (or oil) phase. If the reaction product is hydrophilic, a temperature increase is chosen and the product is recovered from the aqueous phase. If the product is lipophilic, the temperature is instead decreased, and the product is recovered from the excess hydrocarbon phase. The principle has been applied successfully to an HLADH-catalyzed reaction in microemulsion based on C,2E5 [132],... 

The phase behavior of ternary and quaternary systems of the type water-oil-surfactant-cosurfactant is affected strongly by the addition of other components. Therefore, it is questioned how the solubilization of soil during the use of microemulsions as cleaning media in the washing process influences their existence region in the phase diagram and their solubilization power. To test this effect, the temperature dependence of the phase behavior of samples 10-28 (see Table 2) after their use in model... 

In the first case the third phase, a bicontmuous microemulsion, is formed because of temperature-dependent association structures of ethylene oxide adduct surfactants. Fig. 5 (19-21). At low temperatures the surfactant forms micelles in water and the hydrocarbon is solubilized into these micelles (Fig. 5a). 

Based on these studies and on many others with different techniques [see, e.g., (39)] one has obtained a quite detailed knowledge of the properties of the surfacant film separating the oil and water domains. In particular, the value of K is relatively well known (5-10 x 10 ) J) while for k the picture is still unclear. From studies of the temperature variation of the properties of microemulsions the temperature dependence of Hq has been characterized for films. It can be written as... 

As a final comment, let us briefly discuss the permittivity of the studied microemulsions. Figure 22 shows the temperature dependencies of the experimental permittivity and the results of the calculations on the basis of the developed model performed by using Eqs (67) and (71). The difference between the values of e obtained from these formulas can only be observed at high s. The calculated values of s agree well with the experimental data in the region far below the onset of percolation (T < Tqjj), where the assumptions of the model are fulfilled. At temperatures close... 

Figure 21 Temperature dependence of experimental [and calculated on the basis of Eq. (64)] macroscopic apparent dipole moments of a droplet of the AOT/water/decane microemulsions. Experimental values for the dipole moment are shown for various volume fractions (/> of the dispersed phase 0.043 ( 0.13 (O) 0.26 (A) and 0. 39 (V). Calculated values are shown by the solid line. (From Ref 5. With permission from Elsevier Science B.V.)...

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